Internal Breaker for Oilfield Fluids

ABSTRACT

Delayed breakers are given that break viscoelastic surfactant fluids inside the pores of formations into which the fluids have been injected. The breakers comprise proteins, proteins that contain breakers, or cells that contain breakers. Proteins become breakers, and proteins and cells release breakers, due to a triggering mechanism that may be, for example, a change in temperature, pH, or salinity.

CROSS-REFERENCE TO RELATED APPLICATIONS

This application is related to copending application “Internal Breakerfor Oilfield Treatments,” inventors Jesse Lee, Philip Sullivan, ErikNelson, Yiyan Chen, Carlos Abad, Belgin Baser, and Lijun Lin, filed Sep.18, 2006. This application is also related to copending application“Oxidative Internal Breaker for Viscoelastic Surfactant Fluids,”inventors Lijun Lin, Carlos Abad, Belgin Baser, Philip Sullivan, YiyanChen, and Jesse Lee, filed Sep. 18, 2006. This application is alsorelated to copending application “Method for Limiting Leakoff and Damagein Hydraulic Fractures,” inventors Richard Hutchins, Marie Dessinges,and Carlos Abad, filed Sep. 18, 2006. These applications are allassigned to the assignee of the present application and are herebyincorporated in their entirety.

BACKGROUND OF THE INVENTION

The Invention relates to recovery of oil and gas from wells, and moreparticularly to breaking fluids inside formation pores when usingviscoelastic surfactant fluid systems (VES's) as carrier fluids andtreatment fluids.

There are many applications in which breakers are needed to decrease theviscosity of treatment fluids, such as fracturing, gravel packing, andacidizing fluids, viscosified with polymers or crosslinked polymers orviscoelastic surfactants. Most commonly, these breakers act in fluidsthat are in gravel packs or fractures; some breakers can work in fluidsin formation pores. Breakers decrease viscosity by degrading polymers orcrosslinks when the viscosifiers are polymers or crosslinked polymers.Breakers decrease viscosity by degrading surfactants or destroyingmicelles when viscosifiers are viscoelastic surfactant fluid systems.Most breakers are solids, for example granules or encapsulatedmaterials, that do not enter the formation.

There is sometimes a need to break viscous fluids within the pores offormations, for example when viscous fluids enter formations duringfracturing, gravel packing, acidizing, matrix dissolution, lostcirculation treatments, scale squeezes, and the like. Breakers that areeffective inside formations will be called internal breakers here. Thesefluids that enter the formation may be main treatment fluids (such asfracturing fluids) or they may be secondary fluids (such as flushes ordiversion fluids such as viscoelastic diverting acids). Typically it isnecessary that the break be delayed, that is that the breaker not actuntil after the fluid has performed its function.

Compositions and treatment methods using a delayed internal breaker, ora precursor that after a delay releases or decomposes or transforms intoan internal breaker without mechanical or chemical action by theoperator, would be of value. It would be desirable to have a number ofsuch materials so that they could be used under different subterraneanconditions, for example different temperatures and different formationfluid chemistries.

SUMMARY OF THE INVENTION

One embodiment of the Invention is a method of treating a subterraneanformation penetrated by a wellbore involving a) injecting into theformation a fluid containing a non-polymeric viscosifier that acts bycreating a three dimensional structure in the fluid, and a breakerinvolving a protein, and b) allowing the protein to disrupt thestructure. The protein is selected from fibrous proteins (for example,cytoskeletal proteins and extracellular matrix proteins), globularproteins (for example, plasma proteins, coagulation factors,hemoproteins, hormones, DNA-binding proteins, and immune systemproteins), and enzymes. The protein is, for example, egg white oralpha-amylase. In yet another embodiment, the protein is selected fromoxidoreductases, transferases, hydrolases, lyases, isomerases, andligases.

In another embodiment, the non-polymeric viscosifier is a viscoelasticsurfactant, for example a betaine or an amidoamine oxide.

In yet another embodiment, the fluid also contains a sugar, for example,sucrose, d-fructose, or d-sorbitol.

In yet another embodiment, the step of allowing the protein to disruptthe structure is delayed by enclosing the protein in a material selectedfrom fatty acids, polyvinyl alcohols, synthetic resins, phenolic resins,acrylate polymers and copolymers, lactic acid and glycolic acid polymersand copolymers, and mixtures of these materials.

Another embodiment is a method of treating a subterranean formationpenetrated by a wellbore involving a) injecting into the formation anaqueous fluid viscosified with a non-polymeric viscosifier, that acts bycreating a three dimensional structure in the fluid, and a globularprotein that contains a breaker for the viscosifier, and b) allowing theprotein to release the breaker. The globular protein is, for example,selected from albumins, caseins, and glutens. The breaker in theglobular protein is selected, for example, from long chain alcohols,fatty acids, fatty acid esters, mono, di or triglicerides of fattyacids, long chain alcohol phosphate esters, diacid esters, aliphatichydrocarbons, aromatic hydrocarbons, and mixtures of these materials.

Another embodiment is a method of treating a subterranean formationpenetrated by a wellbore involving a) injecting into the formation afluid viscosified with a non-polymeric viscosifier that acts by creatinga three dimensional structure in the fluid, a precursor of a breaker forthe viscosifier (the precursor capable of being degraded by an enzymeinto a breaker for the viscosifier), and an enzyme capable of degradingthe precursor, and b) allowing the enzyme to degrade the precursor torelease the breaker. The precursor is, for example, selected from di ortriglycerides of fatty acids, and phospholipids.

Yet another embodiment of the Invention is a fluid compositioncontaining a non-polymeric viscosifier, that acts by creating a threedimensional structure in the fluid, and a component selected from aprotein, a protein containing a breaker for the viscosifier, and acombination of a precursor capable of being degraded by an enzyme into abreaker for the viscosifier and an enzyme capable of degrading theprecursor.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 shows the effect of a protein breaker of the Invention on theviscosity of a viscoelastic surfactant fluid at 65.6° C.

FIG. 2 shows the effect of a protein breaker of the Invention on theviscosity of a viscoelastic surfactant fluid at 93.3° C.

FIG. 3 shows the effect of a protein breaker of the Invention on theviscosity of a viscoelastic surfactant fluid at 121° C.

FIG. 4 shows the effect of a protein breaker of the Invention on theviscosity of a viscoelastic surfactant fluid at 93.3° C. as a functionof protein breaker concentration.

FIG. 5 shows the effect of various sugars as delay agents on theperformance of a protein breaker of the Invention at 93.3° C.

FIG. 6 shows the effect of a protein breaker of the Invention on theviscosity of a viscoelastic surfactant fluid at 32.2° C.

FIG. 7 shows the effect of a protein breaker of the Invention on theviscosity of a viscoelastic surfactant fluid at 54.4° C.

FIG. 8 shows the effect of a protein breaker of the Invention on theviscosity of a viscoelastic surfactant fluid at 65.6° C.

FIG. 9 shows the effect of a protein breaker of the Invention on theviscosity of a viscoelastic surfactant fluid at 71.1° C.

FIG. 10 shows the effect of a protein breaker of the Invention on theviscosity of a viscoelastic surfactant fluid at 76.7° C.

DETAILED DESCRIPTION OF THE INVENTION

For viscosified fluids used in oilfield treatments, it is important thatthere be a mechanism by which the viscosity can be reduced (that is, thefluid can be broken). Typically, breakers are added to the fluid.Typically, the action of the breaker is delayed or requires a trigger,such as crushing of encapsulated breakers, so that the fluid may performits function before the break occurs. Proper placement is an importantfeature for any breaker; it must be with the fluid that is to be broken.Once a fluid invades a formation, most conventional breakers (such asencapsulated oxidizing agents) cannot clean it up because they form orare incorporated in a filter cake and do not enter the formation. Analternative to including the breaker in the fluid, subsequently addinganother fluid, such as an acid, will be inefficient because of the poorfluid-to-fluid contact. We have found that including certain proteins,for example certain enzymes, in certain forms in a VES-based fluidprovides a delayed break inside formation pores.

The Invention will be described primarily in terms of hydraulicfracturing, gravel packing, acidizing, and fracture acidizing, althoughit is to be understood that the Invention may be used in many otherways, for example many other oilfield treatments. In hydraulicfracturing, most of the injected fracturing fluid contains a proppantsuch as sand or synthetic ceramic beads, so that when the pressure isreleased the proppant is trapped between the fracture faces and preventsthe fracture from completely closing, thus leaving a flowpath open. Theinjected fracturing fluid is normally viscosified. Increased viscosityresults in formation of a wider fracture, thus a larger flowpath. Aminimal viscosity is also required to transport adequate amounts ofproppant; the actual viscosity required depends primarily upon the fluidflow rate, the elasticity of the fluid, and the density of the proppant.In a typical fracturing process, such as hydraulic fracturing withaqueous fluids, the fracture is initiated by first pumping a highviscosity fluid with good to moderate leak-off properties, and typicallyno proppant, into the formation. This initial fluid, typically referredto as a “pad”, is usually followed by a second fluid (fracturing fluid)of similar viscosity carrying an initially low concentration and then agradually or step-wise increasing concentration of proppant into theextended fracture or fractures. The pad initiates and propagates thefracture but does not need to carry proppant. All the fluids tend to“leak off” into the formation from the fracture being created orextended. Commonly, by the end of the job the entire volume of the padwill have leaked off into the formation. This leak-off is determined andcontrolled primarily by the properties of the fluid (and additives itmay contain, such as fluid loss additives or FLA's), the pumping rateand pressure, and the properties of the rock. A certain amount ofleak-off greater than the minimal possible may be desirable, for examplea) if the intention is to place some fluid in the rock to change therock properties or to flow back into the fracture during closure, or b)if the intention is deliberately to cause what is called a “tipscreen-out”, or “TSO”, a condition in which the proppant forms a bridgeat the some point in the fracture, stopping the lengthening of thefracture and resulting in a subsequent increase in the fracture width.In acid fracturing, the fracture fluid is an acid (or other formationdissolving fluid such as a chelant-containing fluid) and the fluidnormally does not contain proppant (although it may); the fracture isheld open by asperities in the fracture faces caused by differentialetching of the formation material. In matrix acidizing, an acid or otherformation dissolving fluid is injected below fracture pressure and thefluid enters the formation and dissolves damaging materials and/or aportion of the formation. Proper leak-off control may be critical to thesuccess of these and other oilfield treatments. In these and many othertreatment types with viscous fluids, after the treatment it is necessaryto decrease the viscosity of the fluids, i.e. to break them, includingbreaking any portion of the fluid that may be in the pores of theformation.

We have found that certain materials can be used as delayed internalbreakers; the break may occur naturally due to naturally changingchemical or physical conditions, for example temperature or pH. Thebreak may optionally be accelerated or delayed if necessary. The breakmay also optionally be triggered by contact with another fluid, such asanother injected fluid, a formation fluid, or a produced fluid.Injecting another fluid to promote the break is not normally desirablebecause of potential costs and complexity, but is within the scope ofthe Invention. All of the degradable materials of the Invention arebreakers for polymer-free (VES) fluid viscosifiers. The internalbreaking effect occurs whether or not a filter cake is also formed bythe addition of a fluid loss additive or by other (solid) breakers.

The Invention is particularly suited for use with polymer free fluids.The Invention is especially useful in gravel packing and the like, wherenear-wellbore damage is often a particularly serious problem. TheInvention makes it possible to treat wells previously eliminated ascandidates for various treatments due to the low fluid efficiency (highleak-off) that would have been expected. The internal breakers may beused as an alternative to some or all of the fluid loss additives thatwould have been used, especially when filter cakes are undesirable;instead of minimizing fluid loss, the fluid loss may be accepted and theleaked-off fluid broken. Viscosified fluids containing internal breakersmay also function as self-destructing diverting agents. They may also beused in kill pills, which can be difficult to break because mechanismsoften available for breaking (such as crushing of encapsulatedmaterials, or later addition of another component) cannot usually beused with kill pills.

In treatments that typically include multiple stages, such as mosthydraulic fracturing, acid fracturing, frac-packing, and gravel packingembodiments, the internal breaker may be added in the pad, throughoutthe treatment or to only some of the stages, such as some of theproppant, gravel, acid, or diversion stages. An internal breaker isparticularly useful in hydraulic fracturing, frac-packing, and gravelpacking because mechanical removal methods are impossible and methodsinvolving contacting the additive with an additional fluid are notalways practical. The compositions and methods of the Invention are alsoparticularly useful in cases where it is desirable to allow a certainamount of treatment fluid to enter the formation, for example for thepurpose of altering formation wettability or oil or water saturation.

Treatment fluids used with the compositions and methods of the Inventiontypically also contain other materials such as demulsifiers, corrosioninhibitors, friction reducers, clay stabilizers, scale inhibitors,biocides, breaker aids, mutual solvents, surfactants, anti-foam agents,defoamers, viscosity stabilizers, iron control agents, diverters,emulsifiers, foamers, oxygen scavengers, pH control agents, buffers, andthe like. Compatibility of the internal breakers of the Invention withsuch additives should be checked in the laboratory. The treatments ofthe Invention are conducted normally; the treatment fluid and additivesare transported to the site, mixed, stored, and pumped in the usual waysfor the respective chemicals. When Resin Coated Proppants (RCP's) areused, testing should be done to ensure that the RCP's and proteinbreakers are compatible and that neither interferes with the performanceof the other; conventional natural and synthetic proppants and gravelsmay normally be used without testing.

The Invention is carried out by considering information about the well,the formation, the fluids and additives available, and criteria for asuccessful treatment, and preparing an optimized plan for maximizingtreatment performance according to the data and the criteria. This isusually done by analyzing the well using treatment design and evaluationsoftware; for example, in hydraulic fracturing software, among otherfactors, pressure gradients are combined with fracture length and heightevolution algorithms, complete leak-off information, and the effects ofmultiple fluid injections and their temperature changes.

The optimal concentration of the internal breaker can be determined bychoosing the desired breaking time and rate and measuring the break withsamples of the intended fluids under the intended formation conditions.Measurement of breaking, and prediction and control of breaking, arefamiliar to those of ordinary skill in the arts of well stimulation,sand control, and other oilfield treatments. A suitable concentration ofthe internal breaker of the Invention is from about 0.05 weight % (ofprotein component) to about 2 weight %, for example from about 0.1weight % (of protein component) to about 1 weight %, for example fromabout 0.1 weight % (of protein component) to about 0.5 weight %. Itshould be understood that throughout this specification, when we list ordescribe a concentration or amount range as being useful, or suitable,or the like, we intend that any and every concentration within therange, including the end points, is to be considered as having beenstated. Furthermore, each numerical value should be read once asmodified by the term “about” (unless already expressly so modified) andthen read again as not so modified unless otherwise stated in context.For example, “a range of from 1 to 10” is to be read as indicating eachand every possible number along the continuum between about 1 and about10. In other words, when we express a certain range, even if weexplicitly identify or refer to only a few specific data points withinthe range, or even to no data points within the range, it is to beunderstood that the inventors appreciate and understand that any and alldata points within the range are to be considered to have beenspecified, and that the inventors have possession of the entire rangeand all points within the range.

If fluid loss additives are used, it is preferable, although notnecessary, to use completely degradable fluid loss additives.Particularly desirable FLA's would be the “internal filter cake/matrixbreaker” materials disclosed in copending U.S. patent application“Internal Breaker for Oilfield Treatments,” inventors Jesse Lee, PhilipSullivan, Erik Nelson, Yiyan Chen, Carlos Abad, Belgin Baser, and LijunLin, filed Sep. 18, 2006. When the pad and the fracture fluid arepolymer-free and any fluid loss additive used is fully degradable,neither the near-wellbore formation nor the proppant bed left in thefracture after the job contains deleterious polymers or solids, as wouldbe the case if the fracture fluid contained any polymer or if the fluidloss additive was not fully degradable. Therefore fracture conductivityis high and skin is low. Similar arguments hold for other treatmentssuch as gravel packing, acidizing and acid fracturing.

Any non-polymeric fluid, for example VES based fluid, that is compatiblewith the formation, the formation fluids, and the other components ofthe fluid, can be used in the Invention. Particularly effectivenon-limiting examples of fluids are those described in U.S. Pat. Nos.5,551,516; 5,964,295; 5,979,555; 5,979,557; 6,140,277; and 6,258,859,all hereby incorporated by reference. Vesicle-based fluids may be used,such as those described in U.S. Pat. No. 6,509,301, hereby incorporatedby reference.

In some cases, a certain amount of leak-off is desired, for example sothat a tip screen-out occurs in fracturing, a condition in which theproppant forms a bridge, preferably at or near the end of the fractureaway from the wellbore, stopping the lengthening of the fracture andresulting in a subsequent increase in the fracture width. For example,hydraulic fracturing followed by gravel-packing in a single operation,sometimes called a frac-pac, fracpac, frac pac, frac and pac, orStimPac, sometimes with a deliberate tip screen-out to generate a shortwide fracture, is usually performed in relatively high permeabilityformations for sand-control purposes. However, such operations aresometimes performed in low permeability formations, occasionally forsand control, but also for other reasons, for example to bypasspermeability damage near the wellbore caused by scaling or to improveupon poor communication between the wellbore and the formation or aprevious fracture, or in formations in which perforating createsdamaging fines, or for other reasons. Such jobs designed to generateshort wide fractures may also be performed without subsequentgravel-packing when sand control is not an issue. The methods of thepresent Invention can be used in any of these cases (fracturing followedby gravel packing and/or fracturing for short wide fractures, in eithercase with or without deliberate tip screen-out).

The acid used in the matrix acidizing and acid fracturing methods ofthis Invention can be any acid used in acid fracturing, includinggelled, self-diverting, and delayed acids. Commonly used, but notlimiting, acids are hydrochloric, hydrofluoric, fluoboric, acetic, andformic acids and mixtures thereof, and those acids in the form of oilexternal emulsions (for reaction rate retardation), or oil internalemulsions (for hydrocarbon solvency). The acids can contain additivessuch as corrosion inhibitors and chelants used to help dissolve rockcomponents and keep them in solution. Gelled, self-diverting, anddelayed acids can be gelled with suitable VES's. Some internal breakersof the Invention may not be compatible with acid, or with strong acid,and laboratory tests should be performed to determine compatibility.

Although in conventional propped fracturing the most common way tocontrol fluid loss is to build an impermeable or reduced-permeabilityfiltercake on the fracture walls (faces), in acid fracturing, especiallywith a low viscosity ungelled acid, pad viscosity is important for fluidloss control. On the other hand, if the acid is viscosified with a VESsystem, then if the VES has higher low-shear viscosity than high-shearviscosity, which is common, then as the VES leaks off a short distanceinto the formation, the flow rate decreases, the shear rate thereforedecreases, and the fluid becomes more viscous. Such effects can reducelow viscosity ungelled or weakly gelled acid leak-off better than awallbuilding system that dissolves or decomposes in acid. In thesecases, an internal breaker would be particularly suitable in the pad.This allows acid treatment a certain selected depth into the formationand the acid then performs the very desirable function of divertingsubsequent acid, after which it is particularly important that the VESsystem then be broken, or flow of fluids will continue to be restricted.Similarly, some internal breakers may be used with viscoelasticdiverting acids, which are acids containing certain viscoelasticsurfactants, such that the fluid has low viscosity as formulated andinjected, but increases in viscosity as the acid reacts with theformation, such as a carbonate. Examples of such viscoelastic divertingacid systems were described in U.S. Pat. Nos. 6,399,546, 6,667,280, and7,028,775 and U. S. Patent Application No. 2003-0119680, all herebyincorporated by reference.

Sometimes acid fracturing is performed with a series of alternating pad,acid, pad, acid, etc. stages in order to optimize coverage. The first,usually but not always non-acidic, pad initiates a fracture for thefirst acid stage to follow. That first acid stage etches a portion ofthe fracture face. Subsequent stages of pad and acid repeat the processuntil the designed treatment volumes have been injected and the desiredfracture has been created. In the past, this process has always used agelled pad, such as one containing a viscoelastic surfactant system, andhas usually but not always used an ungelled acid. The internal breakerof the Invention may be used in at least the first pad and sometimes inall the pad stages, and in any gelled VES acid stages. Similarly, matrixacidizing may be performed with alternating stages of acid and anotherfluid, such as a diverter, some or all of which may be viscosified; theinternal breaker of the Invention may be included in some or all ofeither the acid or the other fluid to break a VES viscosifier. It shouldbe noted that the internal breakers of the Invention may be used forbreaking foams and energized fluids as well as straight fluids.

Proteins have been found to be particularly suitable internal breakers.Enzymes are a particularly useful type of protein for this purpose. Notto be limited by theory, but it is believed that the hydrophobicportions of proteins, for example enzymes, disrupt micelles, such as themicelles that give structure (and therefore viscoelasticity) toviscoelastic fluids made with viscoelastic surfactant systems. At roomtemperature, proteins may be dissolved in water, where they may haveglobular or folded structures that “hide” hydrophobic regions within ahydrophilic shell; this is the “natural” state in water. In this form,they do not interfere with the micellar structure of the VES fluid. Uponheating, however, the protein molecules de-nature and unfold to exposetheir hydrophobic cores. This hydrophobic surface acts to break the VESfluid. Such a process is supported by numerous examples from the modernfood industry in which proteins are heated to a denaturing temperatureto unfold molecules and expose hydrophobic regions. The rate at whichthe protein molecules denature and break the fluid can be controlled bychemistry. This makes this breaker type particularly attractive sincecontrolled breaks can be customized for applications; some addedchemicals slow down the denaturing process. Examples will be shown inwhich sugars were used to control the break time of a VES fluid. Thereis no need to rupture an encapsulated material or add another componentor change the properties (other than temperature) of the fluid. Thisallows the breaker to be used for a wide range of applications includinggravel packing, matrix acidizing, fracturing, and in foamed fluids. Asuitable example of a protein molecule that works in this way isalpha-amylase. In fresh water it is known to denature at temperaturesnear 200° F. (93.3° C.) to expose a hydrophobic core. Importantly, theprotein is essentially inactive as a breaker for VES fluids until thetemperature increases to that at which the protein denatures.

Recently egg white proteins have also been tested as breakers for VESfluids, and have also been found effective as viscosity breakers.Denaturing a protein to expose a surface of the protein molecule canallow for any number of interactions. In a related use of proteins asVES breakers, numerous globular proteins, for example albumins such asegg albumin, milk albumin, blood serum albumin; casein, and wheatgluten, have large internal hydrophobic cavities in which hydrophobicmolecules can placed and thus can be stored in aqueous solutions but notin contact with water. Such hydrophobic molecules, for example longchain alcohols (for example having from about 8 to about 24 carbons),fatty acids (for example having from about 12 to about 24 carbons),fatty acid esters, mono, di or triglicerides of fatty acids, long chainalcohol phosphate esters, diacid esters (also known as dibasic esters)such as dimethyl butanedioate, dimethyl adipate, other dibasic esters,aliphatic or aromatic hydrocarbons, and mixtures thereof, may beselected because they are VES breakers. Such proteins may then be usedas delayed breakers by releasing the hydrophobic entity when the proteinis denatured. In another use of enzymes as delayed breakers, the enzymesmay attack molecules such as di or triglycerides of fatty acids, andphospholipids such as lecithin and cephalin, which then cleave intohydrophobic pieces that are breakers for VES fluids.

Many proteins may be used in the Invention, provided that theirsecondary, tertiary or quaternary structures can be altered (forexample, denatured) by the conditions (for example, pH, temperature,salinity, solvent, or pressure) that the VES fluid will experience. Adenatured protein is one which has lost its functional conformation.Once denatured, a protein loses most, if not all of its biologicalactivity. It is in this non biologically functional form, that theprotein may act as an effective breaker for VES fluids. A protein may bedenatured through various means including exposure to extremes of heat,pH, salt concentration, denaturing agents like urea/guanidine chloride,and exposure to certain detergents. The denatured proteins work aseffective breakers for VES fluids at various conditions and with varyingdegrees of delay, for example as a function of the pH, temperature,salinity, pressure and molecular composition of the protein. Manydifferent proteins are effective: i) Fibrous proteins such as thecytoskeletal proteins, for example Tubulin, Actin, Keratin, and Myosin;and the extracellular matrix proteins, for example Collagen, Elastin andReelin; ii) Globular proteins such as the plasma proteins, for exampleAlbumin, and Serum Amyloid; and coagulation factors, for example Fibrinand Thrombin; and hemoproteins, for example Hemoglobin and Myoglobin;and hormones, for example Oxytocin and Insulin; and DNA-bindingproteins, for example the Histones; and immune system proteins forexample Immunoglobins; iii) Catalytic proteins, for example enzymes, mayalso be used in the Invention.

The Enzyme Commission number (EC number) is a well-known numericalclassification scheme for enzymes, developed by the International Unionof Biochemistry and Molecular Biology; it is based on the chemicalreactions the enzymes catalyze. As a system of enzyme nomenclature,every EC number is associated with a recommended name for the respectiveenzyme. Every enzyme code consists of the letters “EC” followed by fournumbers separated by periods. Those numbers represent a progressivelyfiner classification of the enzyme. The main types of enzymes that maybe used in the present Invention are those in groups EC-1 through EC-6.These groups are: EC 1: Oxidoreductases such as Dehydrogenases,Oxidases, Luciferases, and Reductases; EC 2: Transferases such asTransaminases, and Kinases; EC 3: Hydrolases such as Lipases, Amylases,Peptidases, and Glucosidases; EC 4: Lyases; EC 5: Isomerases; and EC 6:Ligases. Strictly speaking, these EC numbers do not specify enzymes, butenzyme-catalyzed reactions. When different enzymes (for instance fromdifferent organisms) catalyze the same reaction, then they receive thesame EC number. We intend that the above list of EC numbers representsall of the enzymes that fall within the indicated classes and thatcatalyze the indicated reactions.

To delay the break, enzymes and proteins may be encapsulated in, forexample, fatty acids, polyvinyl alcohol, synthetic resins such as epoxyresins, phenolic resins, acrylate polymers and copolymers, andpolylactic acid and polyglycolic acid polymers and copolymers. Thesolubility and/or the porosity of the coating dictate the time dependentrelease of the enzyme. The coating is used to delay the release of theenzyme. The coating thickness and type are chosen as a function of theconditions (temperature, pressure, pH and required time delay) for theapplication of the fluid in the oilfield. The enzyme may be released bydiffusion through the coating prior to its complete dissolution, or maybe released after dissolution, depending upon the effective radius ofthe enzyme and the changing porosity of the coating the duringdissolution process.

The internal breakers of the Invention may be added to a wellbore fluidby metering them in to the base water fluid as a concentrated liquid. Ifthe material is received as an emulsion or dispersion, it can be storedin that form and used in that form directly. If it is received in dryform (for example as a solid dispersible powder of fine polymer beads oras a dry emulsion) the particles can be pre-dispersed in water or brineas required and metered in as a liquid stream, or alternatively they maybe added as solids to the base fluid stream.

The internal breakers of the Invention may also be used in otherindustries such as household and industrial cleaning.

A particular advantage of many the internal breakers of the Invention isthat they and their degradation products are generally not toxic tohumans and aquatic animals and are they are typically biodegradable.

The reactivity of a given internal breaker at a particular temperatureand in contact with a viscosified fluid or fluids of a particularcomposition (for example pH and the concentration and nature of othercomponents, especially electrolytes), is readily determined by a simpleexperiment: exposing the fluid or fluids to the internal breaker undertreatment conditions and monitoring the viscosity.

Although the internal breakers of this Invention may be used with VES'smade with any type of surfactant, or mixtures of surfactants, with orwithout one or more co-surfactants, and with or without other additivesintended to stabilize or modify the properties of the micelles orvesicles (such as buffers, shear recovery additives, salts, and rheologyboosters). Preferred VES's are cationic, anionic, amphoteric, andzwitterionic. Suitable VES's, for example, are described in thefollowing U.S. patents, all of which are hereby incorporated in theirentirety: U.S. Pat. Nos. 5,964,295; 5,979,557; 6,306,800; 6,637,517; and6,258,859. The viscoelastic surfactant may be, for example, of thefollowing formulae: R-Z, where R is the hydrophobic tail of thesurfactant, which is a fully or partially saturated, linear or branchedhydrocarbon chain of at least 14 carbon atoms and Z is the head group ofthe surfactant which may be for example —NR₁R₂R₃ ⁺, —SO₃ ⁻, —COO⁻ or, inthe case where the surfactant is zwitterionic, —NR⁺(R₂)(R₂)R₃—COO⁻ whereR₁, R₂ and R₃ are each independently hydrogen or a fully or partiallysaturated, linear or branched, aliphatic chain of at least one carbonatom; and where R₁ or R₂ may comprise a hydroxyl terminal group.

Cleavable viscoelastic surfactants, for example of the followingformula, may be used, as disclosed in International Patent ApplicationWO02/064945: R—X—Y—Z, where R is the hydrophobic tail of the surfactant,which is a fully or partially saturated, linear or branched hydrocarbonchain of at least 18 carbon atoms, X is the cleavable or degradablegroup of the surfactant which is an acetal, amide, ether or ester bond,Y is a spacer group which is a short saturated or partially saturatedhydrocarbon chain of n carbon atoms where n is at least equal to 1,preferably 2 and, when n is equal to or greater than 3, the chain may bea straight or branched saturated or partially saturated chain, and Z isthe head group of the surfactant which can NR₁R₂R₃ ⁺, —SO₃ ⁻, —COO⁻ or,in the case where the surfactant is zwitterionic, —N⁺(R₁R₂R₃—COO⁻) whereR1, R2 and R3 are each independently hydrogen or a fully or partiallysaturated, linear or branched, aliphatic chain of at least one carbonatom, possibly comprising a hydroxyl terminal group. Due to the presenceof the cleavable or degradable group, cleavable surfactants are able todegrade under downhole conditions.

A nonlimiting example of a suitable cationic viscoelastic surfactantuseful for the implementation of the Invention isN-erucyl-N,N-bis(2-hydroxyethyl)-N-methyl ammonium chloride. Nonlimitingexamples of some suitable anionic viscoelastic surfactants useful forthe implementation of the Invention are monocarboxylates RCOO⁻ such asoleate where R is C₁₇H₃₃ or di- or oligomeric carboxylates such as thosedisclosed in International Patent Application WO 02/11874.

The protein breakers and methods of this Invention have been found to beparticularly useful breakers when used with several types ofzwitterionic surfactants. In general, suitable zwitterionic surfactantshave the formula:RCONH—(CH₂)_(a)(CH₂CH₂O)_(m)(CH₂)_(b)—N⁺(CH₃)₂—(CH₂)_(a′)(CH₂CH₂O)_(m′)(CH₂)_(b′)COO⁻in which R is an alkyl group that contains from about 11 to about 23carbon atoms which may be branched or straight chained and which may besaturated or unsaturated; a, b, a′, and b′ are each from 0 to 10 and mand m′ are each from 0 to 13; a and b are each 1 or 2 if m is not 0 and(a+b) is from 2 to about 10 if m is 0; a′ and b′ are each 1 or 2 when m′is not 0 and (a′+b′) is from 1 to about 5 if m is 0; (m+m′) is from 0 toabout 14; and CH₂CH₂O may also be oriented as OCH₂CH₂. Preferredsurfactants are betaines and amidoamine oxides.

Two examples of betaines are oleylamidopropyl dimethyl betaine anderucylamidopropyl dimethyl betaine. Oleylamidopropyl dimethyl betainecontains an oleyl acid amide group (including a C₁₇H₃₃ alkene tailgroup); erucylamidopropyl dimethyl betaine contains an erucic acid amidegroup (having a C₂₁H₄₁ tail group). Betaine surfactants, and others thatare suitable, are described in U.S. Pat. No. 6,258,859.

Although the Invention has been described throughout using the term“VES”, or “viscoelastic surfactant” to describe the non-polymericviscosified aqueous fluid, any non-polymeric material may be used toviscosity the aqueous fluid provided that the requirements describedherein for such a fluid are met, for example the required viscosity,stability, compatibility, and lack of damage to the wellbore, formationor fracture face. Examples, without regard to whether they form, or aredescribed as forming, vesicles or viscoelastic fluids, include, but arenot limited to, those viscosifiers described in U.S. Pat. No. 6,035,936and in GB application No. 2,366,307A.

Also optionally, fracturing fluids may contain materials designed toassist in proppant transport and/or to limit proppant flowback after thefracturing operation is complete by forming a porous pack in thefracture zone. Such materials can be any known in the art, such as areavailable from Schlumberger under the tradename PropNET™ (for examplesee U.S. Pat. No. 5,501,275). Exemplary proppant flowback inhibitorsinclude fibers or platelets of novoloid or novoloid-type polymers (U.S.Pat. No. 5,782,300).

The choice and concentration of internal breaker and associatedadditives (appropriate delay agents or accelerating agents) is basedprimarily on the desired time before the delayed break, which willdepend upon the choice and concentration of VES and the temperature, andupon the size of the job, the nature of the job, and other factors knownto those of ordinary skill in the art. Suitable choices andconcentrations may be determined by simple laboratory experiments, forexample by mixing all the components, heating to the job temperature,and monitoring the viscosity. A requirement is compatibility of thewater with the VES system and the internal breaker (protein breakers maybe salinity and pH sensitive). The system comprising an internal breakeralso works with VES systems that contain co-surfactants or otheradditives commonly included in oilfield treatment fluids. Again, arequirement is compatibility of the internal breaker, the VES system,and the other components. The fluid containing an internal breaker maybe batch-mixed or mixed on-the-fly.

Any additives normally used in such treatments may be included, againprovided that they are compatible with the other components and thedesired results of the treatment. Such additives can include, but arenot limited to anti-oxidants, corrosion inhibitors, delay agents,biocides, buffers, fluid loss additives, etc. The wellbores treated canbe vertical, deviated or horizontal. They can be completed with casingand perforations or open hole.

In gravel packing, or combined fracturing and gravel packing, it iswithin the scope of the Invention to apply the compositions and methodsof the Invention to treatments that are done with or without a screen.Although treatments are normally done to promote hydrocarbon production,it is within the scope of the Invention to use the compositions andmethods of the Invention in wells intended for the production of otherfluids such as carbon dioxide, water or brine, or in injection wells.Although we have described the Invention in terms of unfoamed fluids,fluids foamed or energized (for example with nitrogen or carbon dioxideor mixtures thereof) may be used. Adjustment of the appropriateconcentrations due to any changes in the fluid properties (or otherparameters, such as proppant concentration) consequent to foaming wouldbe made.

EXAMPLES

Enzymes An example of a suitable protein molecule is alpha-amylase;experiments demonstrate that alpha-amylase progressively breaks a VESfluid. The VES fluid system investigated was made with 6% of aviscoelastic surfactant concentrate containing about 38 weight %erucylamidopropyl dimethyl betaine surfactant, 1.1 weight %polynaphthalene sulfonate, 22 weight % isopropanol, 5 weight % sodiumchloride and the remainder water. As shown in FIG. 1 at 150° F. (65.6°C.), FIG. 2 at 200° F. (93.3° C.), and FIG. 3 at 250° F. (121° C.), VESfluids containing this protein molecule progressively lost viscositywith time at temperature. At the lowest temperature, there was no break,and in fact the VES appeared to have been slightly stabilized; at theintermediate temperature, the break was slow and incomplete; at thehighest temperature, the break was fast, and was complete by the timethe fluid had reached the test temperature. The protein was added as 11weight % of a mixture containing 8 weight % d-sorbitol, 18% sodiumchloride, 72% water, and 1.4-1.5% 1,4-alpha-d-glucan glucanohydrolase.Laboratory work with the addition of only the other ingredients in themixture to the fluid did not show any such break effect, confirming thatthe protein molecule was causing the break. Also, it should be notedthat “natural” alpha amylase, which is an enzyme for starch hydrolysis,has no enzymatic activity for betaine surfactants. In fact, laboratorytesting further showed that when the protein was denatured separatelyfrom the surfactant and then the two components were combined, thedenatured enzyme still broke the fluid.

FIG. 4 shows experiments in which the same VES fluid and varying amountsof the same enzyme concentrate as were used in the experiments justdescribed, were heated at 200° F. (93.3° C.) in a Fann 50 rheometerwhile the viscosity was monitored. It can be seen that with from 5 to 9weight % of the enzyme concentrate, there was a very slow break; with11% of the enzyme concentrate, there was a slow and steady break thattook about 3 to 4 hours.

FIG. 5 shows experiments with added sugars to delay the break. Theseexperiments were done with the same VES fluid as above, and 11 weight %of the same enzyme concentrate, at 200° F. (93.3° C.). It can be seenthat 1 weight % added sucrose slowed the break; 1 weight % addedd-fructose slowed the break a little more; 1% added d-sorbitol slowedthe break even more; and 2% added d-sorbitol slowed the break the most.With the added d-sorbitol, the fluid was almost unbroken after over 7hours.

Egg White The same VES system was used as in the examples above. Theprotein used was dried egg whites obtained as Deb-El™ 100% Dried EggWhites from Deb-El Food Products, Elizabeth, N.J., U. S. A. In a firstset of experiments, 1 weight % dried egg whites was added to the VESfluid and heated to a pre-selected temperature in a Fann 50 Rheometer.At temperatures of 32.2, 37.8, 43.3, and 48.9° C. (90, 100, 110, and120° F.) there was no affect. Typical results are shown in FIG. 6 for32.2° C. (90° F.). FIG. 7 shows the results with 1 weight % dried eggwhites at 54.4° C. (130° F.); it can be seen that there was a verygradual break. FIG. 8 shows the results with 0.5 weight % dried eggwhites at 65.6° C. (150° F.); it can be seen that after a delay of alittle over 1 hour after the system reached temperature, a break beganthat was faster than that shown in FIG. 7. FIG. 9 shows the results with0.5 weight % dried egg whites at 71.1° C. (160° F.); it can be seen thatthe fluid was nearly completely broken by the time the system reachedtemperature. The same can be seen in FIG. 10 for a system with 0.5weight % dried egg whites at 76.7° C. (170° F.). Not shown is that at93.3° C. (200° F.) with 1 weight % egg whites the fluid was completelybroken before the intended temperature was reached.

1-24. (canceled)
 25. A method of treating a subterranean formationpenetrated by a wellbore comprising a) injecting into the formation anon-polymeric viscosifier fluid comprising a viscoelastic surfactantthat acts by creating a three dimensional structure in the fluid, and abreaker comprising a protein having a secondary, tertiary or quaternarystructure which can be altered by a condition that the fluid willexperience in the wellbore, and b) allowing said protein to release saidbreaker and disrupt said structure.
 26. The method of claim 25 whereinsaid condition is selected from the group consisting of pH, temperature,salinity, solvent or pressure.
 27. The method of claim 25 wherein saidstructure is denatured by said condition.
 28. The method of claim 25wherein said viscoelastic surfactant is selected from a betaine and anamidoamine oxide.
 29. The method of claim 25 wherein said proteincomprises egg white.
 30. The method of claim 25 wherein said proteincomprises alpha-amylase.
 31. The method of claim 25 wherein said fluidfurther comprises a sugar.
 32. The method of claim 31 wherein said sugaris selected from sucrose, d-fructose, and d-sorbitol.
 33. The method ofclaim 25 wherein said step of allowing said protein to disrupt saidstructure is delayed by enclosing said protein in a member selected fromthe group consisting of fatty acids, polyvinyl alcohols, syntheticresins, phenolic resins, acrylate polymers and copolymers, lactic acidand glycolic acid polymers and copolymers, and mixtures thereof.
 34. Amethod of treating a subterranean formation penetrated by a wellborecomprising a) injecting into the formation a non-polymeric viscosifierfluid comprising a viscoelastic surfactant that acts by creating a threedimensional structure in the fluid, and a protein having a secondary,tertiary or quaternary structure which will be denatured by a conditionthat the fluid will experience in the wellbore, containing a breaker forsaid viscosifier, and b) allowing said protein to release said breaker.35. The method of claim 34 wherein said condition is selected from thegroup consisting of pH, temperature, salinity, solvent or pressure. 36.The method of claim 34 wherein said viscoelastic surfactant is selectedfrom a betaine and an amidoamine oxide.
 37. The method of claim 34wherein said protein comprises egg white.
 38. The method of claim 34wherein said protein comprises alpha-amylase.
 39. The method of claim 34wherein said fluid further comprises a sugar.
 40. The method of claim 39wherein said sugar is selected from sucrose, d-fructose, and d-sorbitol.41. The method of claim 34 wherein said step of allowing said protein todisrupt said structure is delayed by enclosing said protein in a memberselected from the group consisting of fatty acids, polyvinyl alcohols,synthetic resins, phenolic resins, acrylate polymers and copolymers,lactic acid and glycolic acid polymers and copolymers, and mixturesthereof.